Turn the underlined verbs from active into passive. Do not change the grammar tense. In your answer write the subject and the verb in the right form:

Example: The boys broke all windows in this house.
All windows were broken by the boys.

BREAKING NEWS!
This is Rachel Amber, Channel 5. We are on the air near the South Police Department where they 11) are keeping an unknown man. Here are the details!
Someone 12) had intruded into the City Art Gallery last night before they 12) switched on the alarm system. He 13) opened the back entrance door with a key when the night guards 13) were checking the front door video cameras. The night guards 14) noticed nothing at first. But later one of them 15) found the back door opened and 15) raised the alarm. The guards 16) searched all the building and 16) found a man in the staff room. They 17) called the police. Now the police representative 18) has just informed us that they 18) have arrested the man but they 18) cannot identify him. There is no file on him in the police database. The man keeps silence as if he is deaf and mute. Right now we 19) will show his photo. If someone recognizes him, please dial the number you see on the screen.

The underlined verbs in the passage can be turned from active into passive as follows:

1. The unknown man is being kept by the South Police Department. (11)
In this sentence, the subject "South Police Department" becomes the subject in the passive voice, and the verb "are keeping" is changed to "is being kept" to maintain the same tense.

2. The City Art Gallery had been intruded into last night before the alarm system was switched on. (12)
Here, the subject "City Art Gallery" becomes the subject in the passive voice, and the verb "had intruded" is changed to "had been intruded into" to keep the tense consistent. Additionally, the verb "switched on" is changed to "was switched on" in the passive form.

3. The back entrance door was opened with a key when the night guards were checking the front door video cameras. (13)
In this sentence, the subject "back entrance door" becomes the subject in the passive voice, and the verb "opened" is changed to "was opened" to maintain the same tense. The verb "were checking" remains unchanged.

4. Nothing was noticed by the night guards at first. (14)
Here, the subject "night guards" becomes the subject in the passive voice, and the verb "noticed" is changed to "was noticed" to keep the tense consistent.

5. The back door was found opened and the alarm was raised by one of the night guards. (15)
In this sentence, the subject "back door" becomes the subject in the passive voice, and the verb "found" is changed to "was found" to maintain the same tense. The verb "opened" remains unchanged.

6. All the building was searched and a man was found in the staff room by the guards. (16)
Here, the subject "building" becomes the subject in the passive voice, and the verb "searched" is changed to "was searched" to keep the tense consistent. The verb "found" remains unchanged.

7. The police representative has just informed us that the man has been arrested, but he cannot be identified. (18)
In this sentence, the subject "man" becomes the subject in the passive voice, and the verb "arrested" is changed to "has been arrested" to maintain the same tense. The verb "cannot identify" remains unchanged.

8. Right now, his photo will be shown. If he is recognized, please dial the number you see on the screen. (19)
Here, the subject "photo" becomes the subject in the passive voice, and the verb "shown" is changed to "will be shown" to keep the tense consistent. The verb "is recognized" remains unchanged.

By turning these verbs into the passive voice, we shift the focus of the sentence from the doer of the action (subject) to the receiver of the action. This can change the overall tone and emphasis of the sentence.